Examples of this type of polishing apparatus include a polishing apparatus in which a semiconductor wafer serving as the polishing subject is held in a holder, and the holder holding the semiconductor wafer is rotated relative to a polishing member (polishing head) while contacting the polishing member so that the surface of the semiconductor wafer is polished. With this polishing apparatus for polishing the surface of a semiconductor wafer, extremely precise and uniform polishing is required, and therefore measures such as ensuring that the polishing member changes attitude or the wafer changes attitude in accordance with surface irregularities on the wafer (polishing subject) are taken to maintain an optimum finishing condition at all times.
In a polishing apparatus disclosed in U.S. Pat. No. 6,251,215, for example, a rubber sheet having excellent flexibility is used in a head portion for holding a wafer, and air pressure is applied to the rear surface side of the rubber sheet to push the wafer against a polishing head serving as a polishing member via the rubber sheet. Further, in an apparatus disclosed in Japanese Unexamined Patent Application Publication H10-235555, a head portion holding a wafer is connected to a rotary drive shaft via a ball joint structure, and the head portion is driven to rotate in a swinging fashion via the ball joint portion. By employing structures such as the rubber sheet and ball joint described above, the head portion changes its attitude flexibly in accordance with the rotational accuracy of a surface plate to which the polishing pad is adhered and irregularities in the thickness of the polishing pad so that the wafer is in even contact with the polishing pad at all times, thus enabling uniform surface polishing.
In contrast to the constitutions described above, an apparatus which is constituted to be capable of flexibly varying the attitude of the polishing member side, or in other words the polishing pad, is also known. In this apparatus, for example, a wafer is mounted on a wafer chuck through vacuum suction such that the wafer surface to be polished faces upward (in a face-up state), and the wafer is driven to rotate together with the wafer chuck. A polishing head is provided above and facing the wafer, and the polishing head is constituted by a polishing member to which a polishing pad that contacts the wafer surface to be polished is adhered, a drive plate and a rubber sheet (diaphragm) for supporting the polishing member flexibly, and a head housing formed with an interior space which constitutes a pressure chamber for applying air pressure to the drive plate and rubber sheet.
The drive plate and the outer periphery of the rubber sheet are joined at the lower end outer periphery of the head housing, and the drive plate and rubber sheet are joined to the polishing member at an inner peripheral portion. Thus the interior space of the head housing is covered by the drive plate and rubber sheet, thereby forming the pressure chamber. As a result, the polishing member is supported in the head housing via the drive plate, and receives the air pressure in the interior of the pressure chamber evenly via the rubber sheet. When the head housing is driven to rotate, a rotary driving force is transmitted to the polishing member via the drive plate so that the entire device rotates.
Wafer polishing using a polishing apparatus constituted in this manner is performed by causing the polishing pad to contact the wafer surface to be polished, which is held rotatably in the wafer chuck, while rotating the polishing pad. At this time, the polishing pad, while rotating, performs a reciprocating motion in a horizontal direction to the wafer, and thus the entire surface of the wafer is polished evenly. In a polishing apparatus employing this method, pressure is applied to the device surface (surface to be polished) of the wafer, which rotates together with the polishing head. Moreover, at this time a horizontal reciprocating motion known as a swinging motion is applied to the polishing head such that the entire device surface of the wafer can be polished evenly. Note that a polishing thrust F applied to the wafer by the polishing member (polishing pad) is expressed by Equation (1).F=W+P×S  (1)
Here, W is the deadweight of the polishing member and so on, P is the air pressure inside the head housing (pressure chamber), and S is the pressure-receiving surface area of the polishing member.
Meanwhile, prior to the beginning of the respective polishing operations, the polishing pad is subjected to processing by a tool known as a pad conditioner (dresser) to remove approximately 2 to 3 μm of the pad material surface. This pad conditioner is structured such that a disk having granular diamonds electro-deposited onto the surface thereof is pressed against the polishing pad at a fixed pressure while being rotated, thereby polishing the surface of the polishing pad.
Hence, every time a wafer is polished, approximately 2 to 3 μm of the polishing pad is removed, and therefore, once 500 wafers have been polished, between 1 and 1.5 mm of the polishing pad surface is removed. Naturally, this removal of the polishing pad surface leads to a reduction in the thickness of the polishing pad so that a polishing pad with an initial thickness of 3 mm, for example, is reduced to a thickness between 1.5 and 2 mm after polishing 500 wafers.
When wafers are processed continuously by the polishing apparatus, the thickness of the polishing pad in use is gradually reduced due to the action of the aforementioned pad conditioner. Moreover, it is known that there is variation is the thickness of the wafers to be polished, and although approximately 775 μm is standard for a 300 mm wafer, in actual fact this varies by several tens of μm. It is also believed that the height of the polishing head of the polishing apparatus above the wafer chuck varies by approximately 0.2 mm depending on the thermal expansion coefficient of the metal constituting the apparatus. As a result of these factors, the height of the polishing member to which the polishing pad is adhered in the polishing head varies each time a wafer is processed, and accordingly, the form of a drive ring also varies each time a wafer is processed.
Note that this prior art can be found in Japanese Unexamined Patent Application Publication H11-156711, U.S. Patent Publication No. 6,251,215, and Japanese Unexamined Patent Application Publication H10-235555.
As described above, considering only variation in the thickness of the polishing pad, the maximum variation in the height of the polishing member is approximately 2 mm, and therefore the form of the drive ring varies by a maximum of approximately 2 mm. The drive ring is formed from a thin steel plate, but naturally possesses a modulus of elasticity, and the spring constant of a drive ring that is used often in the prior art, for example, is approximately 520 g/mm. Hence, when the form of the drive ring varies by 2 mm, the calculatory elastic force generated by the drive ring is (520×2=) 1040 g, and this value becomes an error in the polishing thrust F produced by pad thickness variation. Hence, when the thickness of the polishing pad decreases, the polishing thrust F is expressed as in Equation (2), where f is the elastic force generated by the drive ring.F=W+P×S−f  (2)
Further, when the height of the polishing member varies, the rubber sheet deforms. FIGS. 10A, 10B are views showing variation in the form of a rubber sheet 503 (diaphragm) caused by raising and lowering a polishing member 501. In FIG. 10A, an area (the surface area thereof) on the inside of the vicinity of a central portion of recessed portions 504 in the rubber sheet 503 is approximated as a pressure receiving surface (pressure receiving surface area S) for receiving air pressure from the interior of a head housing 502 in the polishing member 501. Note that the polishing member 501 and rubber sheet 503 are formed in disk form, and the pressure receiving surface takes a circular form. A diameter D of the pressure receiving surface (pressure receiving surface area S) is approximated as shown in Equation (3).D≅(ID+OD)/2  (3)
Here, ID is the outer diameter of a contact portion between the rubber sheet 503 and polishing member 501, and OD is the inner diameter of a contact portion between the rubber sheet 503 and head housing 502.
FIG. 10B shows a case in which the polishing member 501 is lowered. In this case, the rubber sheet 503 is lifted from the polishing member 501, causing a phenomenon whereby lifted portions 505 are created in the rubber sheet 503. As a result, the outer diameter ID of the contact portion between the rubber sheet 503 and polishing member 501 changes (decreases) to ID′, and the diameter D of the pressure receiving surface (pressure receiving surface area S) changes to D′. The diameter D′ is approximated as shown in Equation (4).D′≅(ID′+OD)/2  (4)
Hence, when the height of the polishing member varies upon reception of air pressure P from within the head housing, the pressure receiving surface area S of the polishing member varies, and as a result, the linearity of the relationship between the air pressure P in the head housing and the polishing thrust F applied to the wafer by the polishing member is lost.